This is a classic example of a flaw in how this site sorts ideal moves. Poison jab by itself is slightly better than bug bite, but since both give 7 energy per use and bug bite takes less than half the time, a beedrill with bug bite can use its charge attack more often, actually giving it an overall dps of over 19 with sludge bomb, while the dps overall with poison jab + sludge bomb is less than 18. Add the fact that the fast moves allow you to attack more while still dodging and bug bite is far and away the better move for attacking gyms or training.

Just taking the moves that have the biggest dps by themselves doesn't always give you the best overall dps. Likewise, just adding the dps together, like this site does on the ranking page that lists pokemon by best move set, is a terrible indication of actual overall dps.